Make your CoreAnimation applications use the highest available FPS.
Quoting from Apple, CADisplayLink is a timer object that allows your app to synchronize its drawing to the refresh rate of the display. A5 devices (iPhone 4s and iPad 2) are the first to introduce 60 HZ refresh rate - and that the applications can run at its best at 60 frames per second (FPS).
There is a (now-deprecated) property of CADisplayLink called frameInterval that the developers can set to limit the FPS. If set to 1, the FPS is 60. This is true according to the underlying logic of setFrameInterval: method:
Some applications out there choose 2 as a value, rendering the final FPS at 60/2 = 30 which doesn't sound cool for the devices that are capable of higher FPS.
This is where CAHighFPS enforces the value of frameInterval to be 1.
It is a substitute CADisplayLink property of frameInterval (until iOS 15.0), goes by the name preferredFramesPerSecond. If set to zero, the system will try to match the FPS to the highest available refresh rate of the device.
Here's the underlying logic of setPreferredFramesPerSecond::
Again, some applications can explicitly set it to 30 or 60. Those devices that are capable of higher than that will not be so pleased.
This is where CAHighFPS enforces the value of preferredFramesPerSecond to be 0.
Introduced in iOS 15, this is now their main way of dictating the effective FPS. As we want to ensure the maximum FPS, the property preferred and maximum of CAFrameRateRange will be set to the highest supported FPS by the device.
Metal has been a thing since iOS 8. For some reasons, there are not a lot of discussions about optimizing Metal apps for ProMotion display. The best I found are to override -[CAMetalLayer maximumDrawableCount] (reference) and -[CAMetalDrawable presentAfterMinimumDuration:] to allow for ideal ProMotion FPS.
Because CAHighFPS enforces the highest available FPS for the apps, it's only natural that this will consume more energy. Draining may be significant or else. YMMV.